Frequency dividers are often implemented as digital state machines. Alternatives to digital logic dividers include injection-locked frequency dividers (ILFDs) and regenerative dividers (also known as Miller dividers). However, ILFDs typically exhibit a narrow, process-sensitive frequency range. Regenerative dividers are not suitable for a modulus value of 3, which is the focus of this document. For prescaler frequency dividers, the modulus is typically 2 or 3, with higher values being achieved using a cascade of several dividers. A required modulus value of 3 often results from a combination of factors including the frequency of the reference oscillator, the frequency band of operation, and constraints related to a delta-sigma modulator. For example, design constraints of a high-frequency fractional-N phase-locked loop suitable for a 20 GHz chirp synthesizer in an automotive radar environment may require a modulo-3 frequency divider at the output of the voltage controlled oscillator (VCO).
At relatively low frequencies, dividers can be implemented using standard CMOS (complementary metal-oxide-silicon) logic. At input frequencies above, say, 2 GHz, standard CMOS logic does not perform satisfactorily. For high frequencies, faster current-mode logic (CML) is preferred. This preference is particularly acute for digital modulo-3 frequency dividers, which are inherently slower than similar digital modulo-2 frequency dividers. Whereas a modulo-2 divider can be implemented using a single delay flip-flop, a modulo-3 divider requires a chain of two such flip flops and a NOR or NAND gate.
The maximum operating frequency of CML frequency dividers is roughly proportional to the inverse of the total open-loop delay of the CML blocks. Although there are techniques to minimize the CML block delays and thereby increase the maximum operating frequency, they generally incur one or more of the following penalties: (a) low yield due to process variation sensitivity and device mismatch; (b) failure at elevated temperatures; (c) performance sensitivity to variations in supply voltage; and (d) degradation of device reliability resulting in low mean-time-to-failure (MTTF).